The present invention relates to an optical pickup device using an objective lens which is a two-group lens comprised of a first lens and a second lens. More particularly, the invention relates to an optical pickup device in which the distance between the first lens and the second lens is variable and in which the objective lens is driven in focusing and tracking directions.
Conventionally, as a recording medium for information signals, there have been proposed various optical recording media such as so-called optical disks, magneto-optical disks or optical cards. Further, there has been proposed an optical pickup in which light from a light source is beamed onto such an optical recording medium as described above to implement write and read of information signals on a signal recording surface of the optical recording medium. This optical pickup is capable of converging light onto the signal recording surface of the optical recording medium with a reduced beam diameter of the light by enlarging the numerical aperture (NA) of the objective lens, and thus capable of improving the information recording density of the optical recording medium.
In the case of a single lens used as this objective lens, refractive power would be required in an attempt to obtain a high numerical aperture. However, enlarging the refractive power would cause the curvature of the lens surface to decrease, making positioning accuracy between refraction planes stricter. From these and other reasons, the numerical aperture of the single lens has hitherto been limited to about 0.6.
In an optical disk, which is an optical recording medium, if the thickness of a protective layer that protects the signal recording surface deviates from a specified value, spherical aberration largely changes. At the same time, the spherical aberration is proportional to the fourth power of the numerical aperture, neglecting the terms of higher orders. Therefore, with the numerical aperture of the objective lens increased, manufacturing tolerance for the protective layer thickness of the disk would be narrower.
In contrast to this, with the use of a two-group lens, the tolerance for design of the protective layer thickness of the disk can be a wide one while a high numerical aperture is enabled. For this reason, attention has been paid to use of a two-group lens as a high numerical aperture objective lens.
Among pickup devices using a two-group lens as shown above, one described in Japanese Patent Laid-Open Publication HEI 10-255290 is shown in FIG. 6 in its structure.
The two-group lens is comprised of a first lens 102, and a second lens 103 opposed to the first lens 102 with a gap 119 provided therebetween. Further, the two-group lens 120 is equipped with a variable-distance driver 116 for making the distance between the first lens 102 and the second lens 103 variable.
Also, the two-group lens 120 includes, in addition to the first lens 102 and the second lens 103, a generally cylindrical bobbin 118 for holding the first lens 102 with an outer peripheral portion of the first lens 102 fitted thereto, a yoke 110 disposed at a disk-side outer peripheral portion of the second lens 103 with the gap 119 formed against the bobbin 118, a coil 111 wound around an outer peripheral portion of the bobbin 118, a magnet 112 mounted on the yoke 110 and spaced from the coil 111, a spring 113 for mechanically coupling the bobbin 118 and the yoke 110 to each other, and a protective material 115 disposed at an outer peripheral portion of a disk-side surface of the first lens 102.
An electric current, when applied to this coil 111, causes a Lorentz force to be generated in combination with magnetic fields generated by the magnet 112 facing the coil 111. The spring 113 mechanically couples the bobbin 118 and the yoke 110 together, thereby elastically supporting the bobbin 118 against the yoke 110. Also, the spring 113 supports the bobbin 118 so that the bobbin 118 is movable only in the optical-axis direction. Then, in the two-group lens 120, the bobbin 118, the yoke 110, the coil 111, the magnet 112 and the spring 113 constitute a so-called voice coil motor, thereby forming part of the variable-distance driver 116. That is, in the two-group lens 120, the distance between the second lens 103 and the first lens 102 that is elastically supported against this second lens 103 via the yoke 110, the spring 113 and the bobbin 118 with a degree of freedom in the optical-axis direction is made variable by the variable-distance driver 116 driven by the Lorentz force generated between the coil 111 and the magnet 112. In addition, in the optical pickup, the two-group lens 120 equipped with the variable-distance driver 116 is supported by an objective-lens driving actuator so as to be movable in a direction along the optical axis of a laser beam emitted by a semiconductor laser as well as in a direction vertical to the optical axis.
When the lens-to-lens distance of the two-group lens is made variable depending on the protective layer thickness and refractive index as in the above-shown prior art, the center-of-gravity position of the movable part in the objective-lens driver varies in the optical-axis direction.
In this case, when an external force in the tracking direction is applied by the objective-lens driver, there occurs a rolling about an axis vertical to the optical-axis direction and to the tracking direction. Further, whereas a relatively small amount of rolling results for a small moving velocity in the tracking direction, increasing the moving velocity higher causes the rolling amount to increase and moreover resonance of the rolling mode to occur, thus making correct tracking no longer implementable.
Also, since the current to be applied to the variable-distance driver depending on differences in protective layer thickness and refractive index is passed generally in DC manner, there is a further problem of increased electric power consumption.
The present invention having been achieved in view of the above circumstances, an object of the invention is to provide an objective lens for use in an optical pickup, the objective lens being a two-group lens comprised of a first lens and a second lens, in which objective lens the resonance of a rolling mode due to a shift between a point of action in the tracking direction and the center of gravity resulting from driving operation by the variable-distance driver is suppressed within a controllable range so that an accurate tracking can be achieved, and further in which objective lens the value of electric current applied to the variable-distance driver is reduced so that the distance between the first lens and the second lens can be ensured with a simple structure, thus the objective lens having a compact constitution.